When two varieties of a self-fertilizing plant such as rice are to be crossed, it is necessary to first avoid self-fertilization by removing all stamens in a glumous flower just before it flowers. The flower is then fertilized using pollen from the pollen parent variety with which it is being crossed. However, a crossing technique that involves such manual operations is poorly suited for the production of a large quantity of hybrid seed on a commercial scale.
Accordingly, hybrid cultivars are produced by a three-line method which makes use of cytoplasmic male sterility. As used herein, “a three-line method” refers to a procedure that employs a sterile line containing male-sterile cytoplasm, a restorer line having a gametophytic fertility restorer gene, and a maintainer line having the same nuclear genes as the sterile line but lacking sterile cytoplasm. Using these three lines, (i) hybrid seeds can be obtained by fertilizing the sterile line with pollen from the restorer line, and (ii) the sterile line can be maintained by fertilizing it with pollen from the maintainer line.
The male-sterile cytoplasm and fertility restorer genes encoded in the nucleus are employed to commercially produce hybrid seed. Fertility restorer genes are classified as gametic or sporophytic type according to their mechanism of action. In the case of gametic fertility restorer genes, the genotype of the pollen determines whether or not the pollen fertility is restored; known examples include the fertility restorer gene Rf-1 for rice BT-type male-sterile cytoplasm and the restorer gene for maize S-type male-sterile cytoplasm. In the case of sporophytic fertility restorer genes, the genotype of the plant that produces the pollen determines whether or not the pollen fertility is restored; known examples include the fertility restorer gene for rice WA-type male-sterile cytoplasm and the fertility restorer gene for maize T-type male-sterile cytoplasm.
When a hybrid is bred by using a gametic fertility restorer gene, the anther of the hybrid variety shows a 1:1 segregation of pollen carrying the fertility restorer gene and pollen lacking the gene, and so the theoretical pollen fertility is 50%. This is half of the theoretical pollen fertility of 100% for a common variety, and has been of concern as a factor that lowers the stability of seed production in hybrids. In fact, hybrids obtained using rice BT-type male-sterile cytoplasm and the fertility restorer gene Rf-1 are generally known to have a poor cold hardiness, which is thought to be attributable to the low (50%) theoretical pollen fertility.
The following problems are associated with sporophytic fertility restorer genes. Although fertility restoration in rice WA cytoplasm is thought to be imparted by a plurality of fertility restorer genes, the number of such genes and their chromosomal positions have not been identified in detail. Hence, to be used in cross breeding, a restorer line for WA cytoplasm, in addition to having excellent properties such as yield and plant type, must also have the ability, as demonstrated in a seed fertility study on F1 plants obtained after being crossed with a sterile line, to completely restore fertility to WA cytoplasm. Regardless of the excellence of properties other than the fertility restoring ability, if the seed fertility in F1 plants obtained after the restorer line has been crossed with a WA cytoplasmic male-sterile line is incomplete, use as a restorer line will be impossible. Moreover, as noted above, because the number and positions of fertility restorer genes in restorer lines have not yet been precisely identified, it is difficult to improve only the fertility restoring ability while retaining the other properties.
A desire thus exists for a method of preparing hybrid cultivars having a high fertility.
Patent Publication No. 1: Japanese Patent Public Disclosure No. 2002-345485
Patent Publication No. 2: WO 02/014506 A1
Patent Publication No. 3: WO 03/027290 A1
Patent Publication No. 4: WO 02/019803 A1
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